1. Field of the Invention
The present invention relates to an ionizer and a static elimination method for ionizing gas with ions ejected from a discharge electrode and bringing the ionized gas into contact with a target of static elimination in order to eliminate static electricity from the target.
2. Description of the Related Art
In a clean room and the like, conventionally, an ionizer has been used for preventing air from being electrically charged or eliminating static electricity from a target of static elimination. Herein, a discharge electrode induces corona discharge by application of high voltage to generate air ions. The generated air ions are brought into contact with a target of static elimination and the like, so that static electricity is eliminated from the target. Since the air ions are electrically charged, foreign matters such as dust and dirt floating in the air are also prone to be electrically charged. Consequently, the ambient foreign matters such as dust and dirt are prone to be attached to the discharge electrode.
Even when the ionizer is used in the clean room, there still remains a slight amount of foreign matters such as dust in the clean room. Consequently, the electrically charged foreign matters are disadvantageously attached to a tip of the discharge electrode by a principle similar to the principle described above. If the foreign matters are attached to the discharge electrode, a static elimination rate significantly decreases. Moreover, the attached dust and the like gather in a cluster and fall on the clean room. Consequently, there is a possibility that such dust and the like make it difficult to keep the environment of the clean room in a favorable state.
In order to solve the problems, for example, JP 09-017593 A discloses an air ionizing device having a configuration that a tip of a discharge electrode is located inward by a predetermined distance (within 1 mm) with respect to a tip of a nozzle. A rate of sheath gas is set at a rate (not less than 1.0 m/s) which prevents occurrence of inclusion of an air flow at a position near the tip of the nozzle.
In a case where the sheath gas contains no negative gaseous molecules, generated electrons are ejected outside the nozzle in addition to the sheath gas. In a case where the sheath gas contains negative gaseous molecules, generated ions are ejected outside the nozzle. When the discharge electrode is applied with high voltage, an ionic wind is generated at the tip of the discharge electrode, so that a jet stream is generated from the nozzle. However, when the rate of the sheath gas is not less than 1.0 m/s, it is possible to attain a satisfactory seal effect by the sheath gas without such a disadvantage that an induction stream generated by the jet stream causes the inclusion of the air flow at the position near the tip of the nozzle.
On the other hand, JP 2006-040860 A discloses an ionizing device for generating ionized air including ambient air from clean gas emitted from a clean gas emission port which is concentric with a tip of a discharge electrode. A periphery of the discharge electrode is in a substantially open state, that is, no nozzle is present around the discharge electrode. Therefore, even when the nozzle is electrically charged in a single polarity, an electric field at the periphery of the discharge electrode is not weakened, leading to prevention of reduction in amount of ions to be generated. Moreover, the clean gas flows along the tip of the discharge electrode to prevent the foreign matters from being attached to the tip.
When the tip of the discharge electrode protrudes from the clean gas emission port in the clean gas emitting direction, an amount of ionized air to be generated can increase as compared with a case where the tip of the discharge electrode is located inside the clean gas emission port. JP 2006-040860 A describes that a level of the tip of the discharge electrode protruding from the clean gas emission port is fixed based on a balance between the viewpoint of prevention of contamination of the discharge electrode and the viewpoint of the amount of ionized air to be generated.
According to the air ionizing device disclosed in JP 09-017593 A, the sheath gas flows slowly at the rate of about 1.0 m/s, leading to reduction in amount of foreign matters attached to the discharge electrode. However, a satisfactory static elimination effect can not be attained because a static elimination rate decreases. According to the ionizing device disclosed in JP 2006-040860 A, on the other hand, ions are supplied to a target of static elimination with ambient air being included at a position near the discharge electrode. Consequently, there is a possibility that dust and the like collide with and are attached to the tip of the discharge electrode when the ions include the ambient air. As described in JP 2006-040860 A, moreover, since the protrusion level of the tip of the discharge electrode is fixed based on the balance between the viewpoint of prevention of contamination of the discharge electrode and the viewpoint of the amount of ionized air to be generated, the ionizing device fails to enhance both an effect of preventing contamination of the discharge electrode and a satisfactory static elimination effect at a high static elimination rate by the satisfactory amount of ionized air to be generated.
The ionizer is indispensable to attain a satisfactory static elimination effect at a high static elimination rate by a satisfactory amount of ionized air to be generated, and must prevent foreign matters from being attached to the tip of the discharge electrode. The reason therefor is described below. That is, if foreign matters are attached to the tip of the discharge electrode, the amount of ions to be generated decreases, resulting in reduction in amount of ionized air to be generated. Consequently, the satisfactory static elimination effect is not attained because the static elimination rate decreases.